The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating, but when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes at least one coil, a write pole and one or more return poles. When a current flows through the coil, a resulting magnetic field causes a magnetic flux to flow through the write pole, which results in a magnetic write field emitting from the tip of the write pole. This magnetic field is sufficiently strong that it locally magnetizes a portion of the adjacent magnetic disk, thereby recording a bit of data. The write field, then, travels through a magnetically soft under-layer of the magnetic medium to return to the return pole of the write head.
A magnetoresistive sensor such as a Giant Magnetoresistive (GMR) sensor, or a Tunnel Junction Magnetoresisive (TMR) sensor can be employed to read a magnetic signal from the magnetic media. The sensor includes a nonmagnetic conductive layer (if the sensor is a GMR sensor) or a thin nonmagnetic, electrically insulating barrier layer (if the sensor is a TMR sensor) sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. Magnetic shields are positioned above and below the sensor stack and can also serve as first and second electrical leads so that the electrical current travels perpendicularly to the plane of the free layer, spacer layer and pinned layer (current perpendicular to the plane (CPP) mode of operation). The magnetization direction of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetization direction of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering of the conduction electrons is minimized and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. In a read mode the resistance of the spin valve sensor changes about linearly with the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
As the track width is narrowed, the effect of noise (mag-noise) generated by thermal vibrations during magnetization of the free layer on the head signal-to-noise ratio (SNR) become prohibitively large. In the latest TMR heads the improvement in the MR ratio is remarkable, however the accompanying improvement in the head signal to noise ratio has been limited. Since the mag-noise increases proportionally as the playback output increases, the head SNR saturates at some maximum value. Thus, to improve the head SNR along with further miniaturization in the future, a reduction in this mag-noise is very important. Magnetic biasing (domain control) of the free layer is effective in reducing mag-noise.
In addition, the magnetic sensor is located between top and bottom shields, and the distance between these shields defines the gap length. In order to increase the data density by increasing the number of bits per inch of data track, it is necessary to reduce the gap thickness as much as possible. The magnetoresistive sensor must be thinned in order to reduce the bit length. Sensors have included a non-magnetic gap that separates the hard bias structure and sensor from the upper magnetic shield. This non-magnetic gap prevents magnetic coupling between the hard bias structure and the upper shield. Unfortunately in order for this gap layer effectively magnetically de-couple the hard bias structure from the upper shield, it must made thick, and this thickness increases the read gap (e.g. bit length). Therefore, there remains a need for a structure and method for producing such a structure that can maximize signal to noise ratio, while also minimizing the read gap for improved data density.